Method for heat treatment



1969 LARS-GUNNAR BERGH 3,429,055

METHOD FOR HEAT TREATMENT Filed Feb. 12, 1968 I Sheet of 2 I I I I l TIO 20 so 40 so 60 TIME IN MINUTES TIME IN MINUTES mvsmon: LA RS. GUN NARBERGH AT TYS.

2 1969 LARS'GUNNAR BERGH 3,429,055

METHOD FOR HEAT TREATMENT Sheet Filed Feb. 12, 1968 FIGB.

INVENTOR LARS GUNNAR BERGH ATTYS.

United States Patent 01 I 3,429,055 Patented Feb. 25, 1969 Bee 10,102/63 US. Cl. 3413.8 Int. Cl. F26b 3/04; B29j /00 2 Claims ABSTRACT OF THEDISCLOSURE A method for heat treatment of dry fibre boards in which theheating medium is blown perpendicularly against the boards and ismaintained between 160 C. and 235 C. to heat the boards and at the sametime prevent local overheating in random areas due to exothermicreactions occurring within the boards.

The present application is a continuation-in-part of my application No.606,458 filed Dec. 30, 1966 as a continuation of my earlier applicationS.N. 360,302 filed Apr. 16, 1964, both now abandoned.

The present invention relates to a method for the heat treatment of drywood fibre boards or similar material and more particularly thetreatment by hot air at temperatures exceeding 160 C.

An important object of the invention is to prevent a harmful overheatingof the dry wood fibre boards or similar material during the heattreatment.

Other objects and a more detailed understanding of the invention may behad by referring to the following description and claims, in conjunctionwith the accompanying drawings in which:

FIG. 1 is graphical representation showing the relationship between theswelling and the heating time at various temperatures of typical woodfibre boards;

FIG. 2 is a graphical representation showing the relationship betweenthe moisture absorption and the heating time at various temperatures oftypical wood fibre boards;

FIG. 3 is a vertical section through a heat treatment apparatusconstructed in accordance with the present invention; and

FIG. 4 is a longitudinal section taken along the line 4-4 in FIG. 3.

Compressed wood fibre boards, for example hardboards, are heat-treatedor hardened in their dry state in order to impart to them certainproperties of strength, water absorption, swelling, etc. This heattreatment is usually carried out with the aid of hot air, which ismaintained in circulation around the boards. If the heat treatment is toproduce the desired eifect, it must be carried out at a high temperatureabove 160 C. At a temperature of approximately 140 C. an exothermicreaction occurs in dry wood fibre board which gradually increases thetemperature of the board above that of the ambient air.

To prevent the temperature of the board from rising sorption, and thatfor a constant time, both swelling and absorption will decrease withincreasing temperature.

The present invention is based on the discovery that the present highdemands for quality properties in the above stated respects cannot bemet by a conventional treatment in known apparatus, operating withparallel flow of treatment medium along the surface of the boards, Sincethe high temperatures necessary to fulfill the said requirements couldnot be used in this apparatus without the risk of ignition and fire. Therisk of ignition and fire is too great, because a completely uniform airdis tribution is never obtained in said known apparatus, and, withinreasonable pressure limits, only a relatively low heat transmissioncoefiicient between air and boards is attained. The heat transmissioncoefiicient is of decisive importance after the exothermic reaction hasset in and, for control of the temperature gradient in the board, it isnecessary to maintain as low a temperature difference as possiblebetween board and air. The quantity of heat to be supplied for heatingthe board to the required temperature and the quantity of heat whichthen must be removed after the board has been brought to the treatmenttemperature is dependent on the product of the heat transmissioncoefiicient and the temperature difference between board and air. A highheat transmission coefiicient permits a short heating-up time and asmall temperature difference when the board has reached the treatmenttemperature.

In the method of the present invention, a high heat transmissioncoefficient is obtained while a harmful increase in the exothermicreaction occurring in the boards is prevented, by supplying hot air ofregulated temperature in the form of uniformly spaced jets at a highvelocity and substantially at right angles against both sides of theboards. In this manner, with the same pressure loss (10-40 mm. Hg) theheat transmission coefiicient value is approximately five times thatobtained by supplying the air in the form of parallel streams. Myinvention is accompanied by other advantages. It provides a very uniformair distribution across the board, both in the lengthwise and crosswisedirections. This is even more pronounced, if the board is kept in motionat right angles in relation to the discharged direction of the air jetsduring the treatment process; the movement can be continuous,progressive or oscillating. In this manner, local excessive temperaturein the board is avoided. The risk of the occurrence of partial excessivetemperature is especially great at the so-called flash or burr, i.e.,the uneven edge portions arising outside the pressing plate during thepressing. With a treatment according to the invention, it is thereforeadvisable that this flash is removed before heat treatment. Withright-angle blowing, boards have been treated at a temperature of 215 C.for a period of six hours without ignition taking place. In the areaswhere the exothermic reaction occurs, the temperature of the board isthen only about 5 above that of the air. A completely satisfactoryresult was obtained after a treatment time of only 30 minutes, whichmeans that there is an adequate margin of safety as regards ignition.The treatment time can be varied within wide limits depending on theproperties it is desired to impart to the treated board.

This invention is applicable to dry boards, i.e., boards in which thereis no free moisture and in which the hygroscopic moisture is at aminimum. The drawing illustrates an apparatus for carrying the methodinto practice. The apparatus consists of a treatment channel providedwith elements for the transport of the boards and equipped with elementsfor heating and circulation of the treatment medium. Characteristic ofthe apparatus is that air distribution boxes are arranged on both sidesof each transport plane and the walls of these boxes facing thetransport plane are provided with a large number of evenly-spaceddischarged openings for blowing-on treatment medium in the form of jetssubstantially directed at right angles to the dry boards.

In the apparatus shown in FIG. 3, an outer housing 1 of the apparatushas inspection panels or doors 2. Blower boxes 3 and 4 are mounted inthe housing from which air is blown through slots or perforated platesat right angles, or possibly slightly inclined, against the material 11.The material is supported on rollers 5 mounted in bearings 6 and drivenby means of sprockets 7 and chains (not shown). Circulation of air inthe apparatus is produced by a fan 8 driven by a motor 9. When the airstrikes the material, it is deflected sideways and is returned throughthe ducts 17 and 18 to the circulating fan 8. The quantity of heatnecessary for the heating-up period is supplied by heaters 10. Thesecould suitably (with regard to the relatively high air temperature beingused) be supplied with hot oil from a source of heat (not shown).Circulating air can also be heated by means of air from an oil-firedheat exchanger; possibly it is conceivable that flue gases could be useddirectly. During heat treatment, gases are generated, some of which arepoisonous (CO) and the housing must therefore be ventilated. This takesplace through a flue 12 fitted with a controllable damper 13. Thelocation of the fan shown in FIG. 3 provides the advantage that thestatic pressure on the housing of the apparatus is low. It can even benegative in relation to the static pressure outside the housing of theapparatus. This makes the risk of external leakage very small. Theapparatus is supported by a stand 14. FIG. 4 shows that the upper blowerbox, which is located nearest to the material, is fitted with normalperforations 15, and the lower box, which must be located further awayfrom the track due to the rollers 5, is fitted with nozzles 16. Theapparatus may also be fitted with double rollers, if desired, in whichcase both of the blower boxes 3 and 4 are fitted with nozzles.

I claim:

1. A method of heat-treating dry hardboards of wood fibre and the likewhich are subject to localized overheating in random areas due toexothermic reactions occurring within the boards at temperatures aboveC., and are subject to charring at temperatures above 235 comprising thesteps of:

supplying air at a temperature exceeding C. and

below 235 C. where chanring starts, and

directing said supplied air in the form of uniformlyspaced jets at highvelocity substantially at right angles against both sides of the boards,said air serving to heat the boards up to a temperature approximatingthe temperature of the air and serving to cool said random areas oflocalized overheating wherein the board temperature has started to riseabove said air temperature.

2. A method according to claim 1 wherein said dry hardboards have anabsence of free moisture and in which the hygroscopic moisture is at aminimum.

References Cited UNITED STATES PATENTS 835,843 11/1906 Baetz 34l3.8 XR2,591,621 4/1952 Shegda 34160 XR 3,099,541 7/1963 Hildebrand 34-23 XR3,199,213 8/1965 Milligan et al. 34-13.8

KENNETH W. SPRAGUE, Primary Examiner.

U.S. Cl. X.R. 34--23

